January 17, 2013

Lack Of Craters On Titan Makes Moon Look Younger

New research based on observations from NASA's Cassini spacecraft show that Titan, Saturn's largest moon, may look younger than it really is.

While most moons display their age through their thousands of craters, NASA said that Titan's craters are being erased, giving it a 'Benjamin Button' appearance.

"Most of the Saturnian satellites - Titan's siblings - have thousands and thousands of craters on their surface. So far on Titan, of the 50 percent of the surface that we've seen in high resolution, we've only found about 60 craters," Catherine Neish, a Cassini radar team associate based at NASA's Goddard Space Flight Center, Greenbelt, Maryland, said in a statement.

"It's possible that there are many more craters on Titan, but they are not visible from space because they are so eroded. We typically estimate the age of a planet's surface by counting the number of craters on it (more craters means an older surface). But if processes like stream erosion or drifting sand dunes are filling them in, it's possible that the surface is much older that it appears."

She said that this research is the first quantitive estimate of how much the weather on Titan has modified its surface.

Titan is the only moon in the solar system with a thick atmosphere, and the only world other than Earth that is known to have lakes and seas. The rain that falls from Titan's skies is not water, but liquid methane and ethane.

The team compared craters on Titan to craters on Jupiter's moon Ganymede, which is a giant moon with a water ice crust, similar to Titan. Ganymede has almost no atmosphere, and thus no wind or rain to erode its surface.

"We found that craters on Titan were on average hundreds of yards shallower than similarly sized craters on Ganymede, suggesting that some process on Titan is filling its craters," said Neish, who is lead author of a paper, published online in the journal Icarus.

Researchers used the average depth-versus-diameter trend for craters on Ganymede to create stereo images from NASA's Galileo spacecraft. The same trend for craters on Titan was calculated using estimates of the crater depth from images made by Cassini's radar instrument.

The atmosphere on Titan is mostly nitrogen, with traces of methane and other molecules. The source of Titan's methane remains a mystery because methane in the atmosphere is broken down over relatively short time scales by sunlight.

Fragments of methane molecules recombine into more complex hydrocarbons in the upper atmosphere, forming a thick, orange smog that hides the surface from view. Some of the larger particles eventually rain out onto the surface, where they appear to get bound together to form sand.

"Since the sand appears to be produced from the atmospheric methane, Titan must have had methane in its atmosphere for at least several hundred million years in order to fill craters to the levels we are seeing," Neish said..

The team estimates that Titan's current supply of methane should be broken down by sunlight within tens of millions of years, so the moon either had a lot more methane in the past, or is being replenished somehow.

They say it is possible that other processes could be filling the craters on Titan, such as erosion from the flow of liquid methane and ethane. However, this type of weathering tends to fill a crater quickly, then more slowly as its rim gets worn down and less steep. If liquid erosion were responsible for Titan's young appearance, then the team would expect to see a lot of partially filled craters on the moon.

"However, this is not the case," Neish said. "Instead we see craters at all stages; some just beginning to be filled in, some halfway, and some that are almost completely full. This suggests a process like windblown sand, which fills craters and other features at a steady rate."

The team wants to perform more research on Titan in order to find its fountain of youth. So far, the difference in depth between craters on Titan and Ganymede is best explained by filling from windblown sand.